1. Field of the Invention
The invention relates to a process for polishing semiconductor wafers. At least one side of at least one semiconductor wafer is pressed against a polishing plate, over which a polishing cloth is stretched. Thus at least one side is polished, while the semiconductor wafer and the polishing plate move relative to each other. The invention also relates to a device which is suitable for carrying out the process.
2. The Prior Art
Making a semiconductor wafer planar by means of a chemo-mechanical polishing process represents an important processing step in the process sequence for producing a planar, flawless and smooth semiconductor wafer. In many manufacturing sequences, this polishing step represents the last shaping step. Therefore this step decisively determines the surface properties, before the semiconductor wafer is further used. For example the wafer may be used as starting material for the production of electrical, electronic and microelectronic components. The objectives of the polishing process are to achieve a high planarity and parallelism of the two sides of the wafer, and to remove surface layers which have been damaged by pretreatments ("damage removal"). An additional objective is to reduce the microroughness of the semiconductor wafer.
Single side polishing processes and double side polishing processes are generally used. In the case of single side polishing of a batch of semiconductor wafers ("single side batch polishing"), one side of the semiconductor wafers is mounted on the front side of a carrier plate. This is accomplished by producing a form-fitting and force-fitting connection between the side of the wafer and the carrier plate. This connection can be by adhesion, bonding, cementing or the application of vacuum. As a rule, the semiconductor wafers are mounted on the carrier plate in such a way that they form a pattern of concentric rings. After having been mounted, the free sides of the wafers are pressed against a polishing plate, over which a polishing cloth is stretched. This pressing is with a defined polishing force and with a supplied polishing abrasive, such that these sides are then polished. In the process, the carrier plate and the polishing plate are usually rotated at different speeds. The polishing force required is transmitted to the rear side of the carrier plate by a pressure piston, or polishing head. A large number of the polishing machines used are designed in such a way that they have a plurality of polishing heads. Accordingly, they are able to accommodate a plurality of carrier plates.
In double side polishing (DSP), the front side and the rear side of the wafers are polished simultaneously. Thus a plurality of semiconductor wafers are guided between two, an upper and a lower, polishing plates over which polishing cloths are stretched. In this embodiment, the semiconductor wafers lie in thin wafer carriers. These carriers are referred to as rotor disks and are also used in a similar form during lapping of semiconductor wafers. Double side polishing processes and devices are always designed for treating groups of semiconductor wafers ("batch polishing").
A plurality of factors make it difficult to achieve the desired planarity and parallelism of the semiconductor wafers, referred to below as the desired geometry. Polished semiconductor wafers often have sides which are not parallel to one another, but rather have the cross-sectional shape of a wedge.
The shape of the wedge can be described using the term "linear thickness variation." The linear thickness variation is the largest measured difference in thickness between two measurement points which lie on the same diameter, symmetrically with respect to the center of the semiconductor wafer. Usually, the measurement points lie symmetrically on a circle which is at a distance of, for example, 6 mm from the edge of the semiconductor wafer. If the edge of the semiconductor wafer which faces toward the edge of the carrier plate is thicker (thinner) than the wafer edge which faces toward the center of the carrier plate, this is known as a positive (negative) linear wedge shape.
Another measurement of the wedge shape of semiconductor wafers is the so-called TTV value (TTV=total thickness variation). This value gives the difference between the thickest and thinnest points on the semiconductor wafer.
A semiconductor wafer wedge shape caused by the polishing is ultimately the result of uneven removal of material. This may arise if the carrier plate is deformed radially during polishing as a result of its own weight or has a certain radial wedge shape caused by its production. Sometimes, incipient wear to the polishing cloth is also a cause of the wafer geometry deteriorating during a number of polishing runs. A certain fundamental wedge shape results even when using carrier plates which are of ideal planarity. This may result from the kinematic ratios during single side (wafer) polishing, which require inhomogeneous removal of material.
EP-4033 A1 discloses inserting interlayers of soft, elastic bodies between the polishing head and the rear side of the carrier plate. This has the result that the carrier plate is deliberately curved slightly in a radially symmetrical manner. In this way, it is partially possible, to prevent the semiconductor wafers from being polished into a wedge. However, this process cannot be automated and is susceptible to failure. This is because its success depends on the experience of and the care taken by the operating personnel. These personnel have to select and insert the interlayers on the basis of their width. However, even if no mistakes are made in doing this, the wedge shape of the polished semiconductor wafers remains above a defined limit value.